ABO3 perovskite-type metal oxides such as lead zirconate titanate (hereinafter also referred to as “PZT”) are commonly used as piezoelectric ceramics. Since PZT contains lead as the A site element, its impact on the environment has been a concern. Accordingly, piezoelectric ceramics composed of lead-free perovskite-type metal oxides are in demand.
NPL 1 reports that dissolving a small quantity of barium titanate in an anti-ferroelectric material sodium niobate gives ferroelectric sodium niobate. NPL 1 also discloses remanent polarization, coercive electric field, piezoelectric constant, and electromechanical coupling coefficient of sodium niobate having a barium titanate concentration of 5% to 20% and sintered at a temperature of 1200° C. to 1280° C. The material disclosed in NPL 1 is not only free of lead but also free of potassium which generates poor sinterability and causes low moisture resistance. The Curie temperature of the material disclosed in NPL 1 is higher than the Curie temperature (110° C. to 120° C.) of barium titanate, which is a representative example of lead-free piezoelectric materials. NPL 1 also discloses that the Curie temperature of the composition (Na0.9Ba0.1)(Nb0.9Ti0.1)O3 with which the highest piezoelectric constant d33=143 pC/N is yielded is 230° C.
PTL 1 discloses that adding cobalt to a piezoelectric ceramic which is a solid solution of sodium niobate and barium titanate improves the piezoelectric constant. PTL 1 also discloses that some samples of the piezoelectric material were difficult to polarize due to a low insulating property of 106Ω or less.